A conventional developing unit using a liquid developer has a structure in which, as shown in FIG. 17, an electrically conductive sheet called a bias blade is disposed on a developing roller and adapted to be biased (see International Patent Application Laid-Open No. WO 01/88630 A1).
As shown in FIG. 18, which depicts the bias blade of FIG. 17 and its associated components in detail, one end of the bias blade is held by a bracket of metal, etc., and the other end of the bias blade is in contact with a liquid developer bearer body in the form of, for example, a developing roller or belt. The bias blade serves to separate the toner layer into a toner cohesion layer that is rich in toner and has a high viscosity, and a quasi-prewet layer that is rich in carrier, by means of electrically migrating the toner particles under application of a bias voltage to the toner layer previously formed on the developing roller. The force causing this toner separation is derived from an electric field induced by the applied bias voltage and is attributed to the electrical migration phenomenon in which toner particles; i.e., electrically charged particles, move under influence of the electric field.
As is apparent from its function, the bias blade must exhibit not only flexibility for holding the toner layer in position and allowing the toner layer to pass through, but also electrical conductivity for inducing an electric field. In terms of flexibility in particular, the type of bias blade to be employed must be determined by comparison with the hardness of the developing roller, but in practice the type of bias blade is selected in view of whether the bias blade allows the toner layer to pass through.
Conventionally, a bias blade is manufactured by molding and cutting a relatively thin sheet of rubber or plastic into a desired shape and must be adjusted in electrical resistance with respect to the resistance of the developing unit. Further, a bias blade must allow a very thin toner layer to pass through, depending on the thickness of the layer of a liquid developer to be used. Generally, the amount of liquid that is allowed to pass depends on applied mechanical pressure, viscosity, and speed. For example, when an ordinary elastic rubber is used, the bias blade has the following relation. The amount of liquid allowed to pass is 20 im in the case where roller rubber hardness: 40 (JIS-A); oil viscosity: 20 cSt; and speed: 250 mm/s.
Furthermore, in terms of the function of a bias blade, a bias blade must have a shape designed so as not to rupture or separate toner, in order to prevent formation of any rivulet. A rivulet refers to a stripe-shaped non-uniform layer (irregularity) which is formed as a result of derangement of a layer of a liquid developer attributable to viscosity of toner when the liquid developer layer is broken and separated at the exit of the blade. When such a rivulet is formed, image quality deteriorates or the image encounters fogging. Consequently, instead of a tip-side edge of a blade, a belly portion of the blade adjacent to the tip-side edge must be pressed against the developing roller. For this purpose, the bias blade assumes the shape shown in FIG. 18.
As described above, there has been desired a method for stably applying an electric field to the toner layer so as to maintain passage of a proper amount of toner without breakage and separation of toner, which would result in formation of a rivulet. Further, since the bias blade utilizes an electrical migration phenomenon, the length of contact between the blade and the developing roller (the length of contact in the moving direction: the nip width) must be increased in order to increase a period of time for movement by migration. However, when the manner of contact of the bias blade is determined so as to suppress breakage and separation of toner at the tip end to thereby prevent formation of rivulets, in many cases, it becomes difficult to increase the nip width of the blade.
Conventionally, a blade is positioned in such a manner that the blade comes into contact with a toner layer in an area starting from a belly portion of the blade to a point very close to the tip-side edge thereof. Therefore, adjustment of the position of the blade has been difficult. When the tip-side edge of the blade comes into direct contact with a toner layer, toner is not permitted to pass through such a contact zone and is scraped off by the blade, which results in formation of a stripe in a developed image. In contrast, when the blade is separated from the toner layer at a position spaced too far from the tip-side edge, formation of a rivulet occurs.
Further, conventionally, high voltage is supplied from a power source to the bias blade, and therefore, means for limiting current must be provided in order to protect the developing roller.
Japanese Patent Application Laid-Open (kokai) No. H7-287450 discloses an alternative method for disposing a bias-applied electrode in opposition to a developing roller. In the method disclosed in this publication, a rigid electrode having a cylindrical inner surface is disposed so as to face the developing roller with high precision. However, in the case in which a toner layer has a thickness on the order of 10 im, a very small gap corresponding to such a thin layer is very difficult to form through only mechanical machining.
In addition, as described above, a liquid toner is applied onto a developing roller, and a latent image on a photosensitive body is developed by use of this liquid toner. However, because of its high viscosity and high concentration, the liquid toner often fails to be uniformly applied onto the developing roller unless a sufficient amount of liquid toner is uniformly conveyed and applied to the developing roller.
FIG. 19 is an illustration showing a conventional construction for feeding a high-viscosity, high-concentration liquid toner. A liquid toner is supplied from a feed tray to a developing roller via a toner applicator roller (a patterned roller). If a patterned roller having an engraved pattern of cells is employed as the toner applicator roller, excessive toner is scraped off the patterned roller by a scraper blade. By virtue of this construction, if the entire developing unit is tilted or if excess toner is supplied, the toner would be prone to leak, thereby contaminating the unit, a printing medium, etc.
Consequently, the present applicant previously proposed a liquid toner supply arrangement as shown in FIG. 20 (Japanese Patent Application No. 2001-77440). A liquid toner is supplied onto a surface of a developing roller from a patterned roller whose circumferential surface is moving in the same direction as the developing roller's circumferential surface (i.e., in the forward rotational direction) at the contact zone. Because the liquid toner is conveyed with the assistance of the circumferential grooves on the pattern roller, a constant amount of toner can be applied, which amount is restricted by only the number and size (cross-sectional area) of the grooves. The toner applied to the patterned roller is supplied from the toner feed tray.
As described above, the patterned roller is disposed in such a positional relation with the toner feed tray as to close its open side. Further, a scraper blade is disposed on the patterned roller at a position downstream of the toner feed tray and is normally pressed against the patterned roller under constant pressure by means of the resilience of a spring, so that the toner can be conveyed and applied onto the developing roller uniformly.
In this illustrated conventional construction, the gaps between the patterned roller and nearby parts are tightly covered by means of seals (illustrated seals 1 and 2) from all sides. Practically, a toner vessel is statically sealed by a cylindrical casing, sponge rubber, etc. Although this method enables provision of a structure which can prevent leakage irrespective of attachment angle, a rubber member for sealing comes into contact with portions other than the scraper blade and, therefore, cohesion of toner occurs at these portions. This toner cohesion causes variation in toner concentration, with the result that the image suffers irregularity and stripes, thereby deteriorating image quality.
The developing process will now be described in more detail. The proper amount of toner to be applied onto the developing roller is determined in terms of volume of the engraved cells of the patterned roller. Notably, the amount of toner is determined by the pressure of contact and the shape of cells; however, in general, not all the toner in the cells is fully transferred to the developing roller. Namely, a certain part of toner fails to be transferred and remains in the cells of the patterned roller after these cells have passed the developing roller. Although this does not pose a serious problem, the toner concentration is apt to change or the residual toner is apt to be scraped off subsequently when the toner comes into engagement with the sealing materials, etc. that are provided for tightly closing the above-described gaps. Either problem can be eliminated when the patterned roller passes the standing toner in the toner feed tray. In actuality, this phenomenon does not occur while the patterned roller is rotating at relatively low speed.
Nonetheless, when a sufficiently high printing speed is required, the circumferential speed of the patterned roller also inevitably becomes higher. In such a case, since the period of time for the patterned roller to pass through the standing toner is short, the above-described phenomenon cannot be completely eliminated; consequently, the resulting image still suffers irregularity and stripes.
As described hereinabove, when sponge rubber and sealing rubber used for constituting a closed structure come into engagement with the patterned roller, solid components of tone cohere locally, with the result that intended uniform application of toner cannot be achieved by means of the scraper blade.